Adjusting Freestyle in the Blender compositor
Yes, it's possible. I mostly render my freestyle stuff on a separate Freestyle-Renderlayer and recombine it with the ordinary one in the Compositor. If you just want to rerender your FreeStyle strokes you can just render the Freestyle Renderlayer.

I've only done it in Cycles so far, so the following is only for Cycles.

To separate it:


Activate Freestyle in Render Settings > Freestyle.
Tick "Transparent" in Render Settings > Film to have the background transparent.
Create a new Renderlayer (eg. "FreestyleLayer").
Create the Freestyle settings you want.
Override samples with 1 (for example).
Turn off "Use Environment", "Use Surfaces" and "Use Hair" in the Layer panel. That way basically nothing gets rendered except the Freestyle strokes.


In the ordinary RenderLayer:


Delete the Freestyle LineSet so you don't get any strokes here.
Activate the "Environment" pass, so we can readd the background if we want to (because we just made it transparent above).




So far for separating. We now have a renderlayer that's transparent and only the Freestyle stroke will be visible.

To recombine it, do it like this in the Compositor:



Note that the Environment Pass is also readded here. This can just be left out if one doesn't want it.

Hope this helps.